Automatic volume control circuit



D. G. BURNSIDE 2 Sheet-Sheet 1 AUTOMATIC VOLUME CONTROL CIRCUIT 2 70FOLLOW/N6 7'0 m/ok TUBES- a j e To GR/DS OF PRIOR TUBE-5' Filed April18, 1959 April 8, 1941.

70 PRIOR TUBES 70 69/05 0F PR/OP. TUBE IN VENTOR. 0 6. BURNS/DE p 1941-i D. G. BuRNsmE 2,237.409

AUTOMATIC VOLUME CONTROL CIRCUIT Filed April 18, 1939 2 Sheets-Sheet 270 FOLLOWING EMISSION au/mEA/r 1'; (MICRO-AMPS) 40 45 .50 F/LAMENTCURRENT- I f (MIL Ll-AMPS) INVEN TOR. DON (I. BURNS/DE ATTORNEY.

Patented Apr. 8, 1941 AUTOMATIC VOLUME CONTROL CIRCUIT Don G. Burnside,East Orange, N. J., asslgnor to Radio Corporation of America, acorporation of Delaware Application April 18, 1939, Serial No. 268,465

8 Claims.

My present invention relates generally to automatic amplificationcontrol, and more particularly to automatic volume control (AVC)circuits adapted to produce control voltage without rectificationofsignal waves.

In the past there have been devised many types of circuits capable ofcontrolling signal amplification automatically. Most of these circuitshave been designed to provide the gain control voltage by rectificationof the signal voltage. While there have been proposed circuits for gaincontrol wherein rectification of carrier voltage is not depended upon,yet such methods have not been as simple as is desirable.

It may be stated that it is one of the main objects of my presentinvention to provide an automatic amplification control circuit whereinthe control voltage is derived from variations in emission current of anelectron discharge device; variations in emission being dependent uponthe amplitude of a radio frequency voltage which is being amplified.

Another important object of this invention is to provide an automaticgain control circuit in a signal tranmission system, the control voltagebeing developed across an impedance located in the emission current pathof an electron discharge device, and the emission current being variedby the application of signal voltage to the emission element of theelectron discharge device.

Still another object of the invention may be stated to include theutilization of a radio frequency, or carrier, voltage to heat thefilament of a diode, or even a triode or other multi-element tube, and asteady voltage being employed to cause the diode current to fiow througha load resistor thereby to provide the required gain control voltage forthe signal grids of the amplifier tubes under control.

Yet other objects of this invention are to improve generally automaticvolume control circuits of the type employing a diode as the gaincontrol device, and more especially to provide a control circuit whereinthe signal voltage is employed to vary the diode filament heating, thecontrol circuit being efficient and reliable in operation, andeconomically manufactured and assembled in radio receiving systems.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawings in which I have er supply transformer ofa radio receiver.

indicated diagrammatically several circuit organizations whereby myinvention may be carried into effect.

In the drawings:

Fi 1 illustrates one gain control circuit embodying the invention,

Fig. 2 shows a modification of the invention,

Fig. 3 shows still another modification,

Fig. 4 graphically shows the relation between filament current andemission in the gain control device.

, Referring to the accompanying drawings, wherein like referencecharacters designate similar circuit elements, Fig. 1 shows a signalamplifier tube I whose cathode is at ground potential,

while the signal grid 2 thereof is connected to the high potential sideof the resonant signal input circuit 3. The latter can be tunable, as inthe case of the tunable radio frequency amplifiers prior to the firstdetector of a superheterodyne receiver. However, circuit 3 can be thefixedly tuned input network of the intermediate frequency amplifier ofsuch a receiver. Again, the circuit 3 may be coupled to one or moreprior tuned amplifiers of the same type, and the receiver can be of thebroadcast sound reproduction type, or it may be a television receiver.Assuming that the receiver is of the superheterodyne type, and thatamplifier I is in an intermediate frequency amplifier, the plate circuit4 will be fixedly tuned to the operating intermediate frequency. Theamplified energy may be utilized in further amplifiers, demodulated, themodulation voltage further amplified and finally reproduced.

In order to maintain a substantially constant carrier amplitude at thedemodulator input circuit, it is, of course, necessary to vary the gainof the pro-demodulator stages in such a manner that substantiallyconstancy at the demodulator input circuit is attained. The diode device5 is provided to act as the gain control element, The signal energy, atradio or intermediate frequency, is impressed upon the diode byconnecting the plate of tube I to the diode filament B by a condenser I.order of mmf. (micromicrofarads), or less. The radio frequency path isthen through the diode filament 6 and capacitor 8 to ground. The

transformer T, whose primary winding is connected to an alternatingcurrent source, has its secondary in series in the diode filamentcircuit. Hence, T acts as the filament heating means, and may comprise,if desired, a tap on the usual pow- A sep- The latter has a capacitanceof the arate winding on the supply transformer may also be used. Ofcourse, a battery may be employed to heat the filament, where thisexpedient is desirable.

The radio frequency choke coil 9 is inserted in series with condenser 1and the transformer primary to prevent radio frequency current frompassing to ground through the winding. If desired, the condenser 1 canbe additionally employed to resonate the coil of output circuit 4 to theoperating frequency, and in such case the usual shunt condenser can beomitted. The diode plate It! is connected to filament 6 through a pathincluding load resistor R in series with current source II. The positiveterminal of source II is grounded. The positive terminal of the latteris connected to the resistor so that a steady direct current voltage ofpositive polarity is applied to the diode plate. The signal grid 2 ofamplifier tube I is connected by lead I2, which includes an alternatingvoltage filter l3, to the plate end of load resistor R. The lead [2applies the direct current voltage developed across resistor R to thevarious signal grids of the controlled amplifier tubes. The lead is theautomatic volume control (designated on the drawing as AVC) connection.

In operation, the filament 6 is left at such a temperature by thevoltage from source T that the steady voltage from source ll causes acurrent to flow of about the magnitude indicated by point a on theFilament current-emission cur rent characteristic shown in Fig. 4. Thisnormal, or initial, emission current flowing through load resistor Rprovides a normal, or maximum amplification, bias for the signal gridsconnected to lead l2. This eliminates the necessity of providing anauxiliary source of grid bias.

Upon the impression of radio frequency voltage upon diode 5, a currentflows through the filament 6 which adds, in a more or less complexmanner, to the already present current from T. This augmented currentflow causes an increase in the temperature of filament 6; an increase inthe electron emission from the filament results. The curve in Fig. 4shows the manner in which emission current increases as the filamentcurrent rises. As the emission current flow through resistor Rincreases, the control voltage applied through lead l2 increases and ina negative, or gain-reducing, sense. By way of illustration, and in noway restrictive, it is pointed out that a diode can be employed at 5which is designed for a filament voltage of about 0.5 and a filamentcurrent of about 40 ma. (milliamperes). An increase of the currentthrough the filament of about 2 or 3 ma. causes a large increase in theemission. Fig. 4 is a curve of the emission characteristic of a diode ofthis type.

In Fig. 2 there is shown a method of avoiding the use of the choke coil9 of Fig. l. The circuit 4 is coupled to the diode 5, in thismodification, by a coil split into substantially equal sections 20, 2|.The filament leads are connected to the outer ends of the coil sections,while the inner ends of the latter are connected to the ends of thesecondary of transformer T. The carrier by-pass condenser 22 isconnected in series relation to the coil sections 20, 2 I, but is inshunt relation to the secondary of transformer T. The negative terminalof source II is connected to the midpoint of the transformer secondary.Otherwise the circuit is similar to the arrangement of Fig. 1. Theamplified signal energy canbe derived by the auxiliary coil 30.Condenser 22, in Fig. 2, is considered to be at practically zeropotential with respect to ground. No radio frequency voltage will appearin the leads from transformer T. The voltages induced in coil sections20 and 2| are assumed to be equal in magnitude.

Another modification of the invention is illustrated in Fig. 3, whereinthe diode filament heating source T is utilized for the additionalfunction of providing the diode anode voltage. Thus, a filament lead isconnected to an intermediate point of the secondary of T, and the diodeanode I0 is in series with the load resistor R as before. The coil endof the load resistor R is at ground potential, while the load isby-passed by a condenser 40. Condenser at has a fairly large capacity tosmooth out the I20 cycle ripple from transformer T. In this circuitarrangement diode 5 rectifies only the low frequency supply voltage, andnot carrier voltage. A filter F is desirable to prevent any lowfrequency voltage from reaching the signal grids of the controlled tubesand causing hum modulation of the carrier voltage. The operation of thisarrangement is similar to that described in connection with Fig. 1.

While I have indicated and described several systems for carrying myinvention into efiect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaims.

What I claim is:

1. In a signal receiver of the type including at least one amplifier ofsignals and a diode device, means applying signals to be amplified uponsaid amplifier, means establishing emission current flow from the diodefilament, means deriving a uni-directional voltage from the normalfilament current, means controlling the gain of the amplifier with saiduni-directional voltage, and means varying the emission from the diodefilament with amplified signal voltage.

2. In a signal transmission system, an automatic transmission controlnetwork comprising an electron discharge device which includes at leastan electron emission element and a cold electrode, means for energizingsaid emission element to provide an electron stream to said coldelectrode, an impedance in circuit with the cold electrode and emissionelement for developing a uni-directional voltage from the electron flowthrough the impedance, means for utilizing the uni-directional voltageto control the transmission through said transmission. system, and meansfor impressing signal voltage upon said emission element for varying theenergization of the latter thereby to vary the magnitude of saiduni-directional voltage.

3. In a radio receiving system of the type including at least one signalamplifier, a diode provided with a filament and an anode, means forheating said filament to provide emission current to said anode, animpedance connected between the filament and anode for developing adirect current voltage from said emission current flow,

means for applying the uni-directional voltage to said amplifier tocontrol the gain thereof. means for applying signal voltage to the diodefilament to vary the heating of said filament over a sufficiently widerange substantially to vary the magnitude of said direct currentvoltage.

4. In a radio receiving system of the type including at least one signaltransmission tube, a

gain control circuit comprising an electron discharge device providedwith at least a cathode and a cold electrode, means for heating theoathode to provide emission current to said cold electrode, an impedanceconnected between the cathode and cold electrode for developing a directcurrent voltage from the emission current, means for applying the directcurrent voltage to a gain control electrode of said transmission tube,means for applying signal voltage to said control device cathode to varythe heating of said cathode sufficiently to control the magnitude ofsaid direct current voltage.

5. In a radio receiving system of the type including at least one signaltransmission tube, a gain control circuit comprising an electrondischarge device provided with at least a cathode and a cold electrode,means for heating the oathode to provide emission current to said coldelectrode, an impedance connected between the oathode and cold electrodefor developing a direct current voltage from the emission current, meansfor applying the direct current voltage to a gain control electrode ofsaid transmission tube, means for applying signal voltage to saidcontrol device cathode to vary the heating of said cathode sufficientlyto control the magnitude of said direct current voltage, said heatingmeans additionally being connected to said cold electrode to provide apositive potential therefor.

6. In an alternating current transmission system, an electron dischargedevice provided with at least an electron emission element and anadditional electrode, said emission element having a normal electronemission, means deriving a unidirectional voltage from space currentflow or said device, means responsive to said voltage for controllingtransmission of current through said system, and means applyingalternating current transmitted through said system to said emissionelement for varying said normal emission.

7. In a high frequency current transmission system, a space dischargedevice provided with an electron emission element and a cold electrode,means establishing a normal flow of electrons between said element andcold electrode, means applying high frequency current from said systemto said element to energize the latter thereby to supplement theelectron flow therefrom, and means responsive to the space current flowof said device for controlling current transmission through said system.

8. In a high frequency current transmission system, a space dischargedevice provided with an electron emission element and a cold electrode,means establishing a normal flow of electrons between said element andcold electrode, means applying high frequency current from said systemto said element to energize the latter thereby to supplement theelectron flow therefrom, and means responsive to the space current flowof said device for controlling current transmission through said system,said energizing means additionally establishing said cold electrode at apositive potential relative to said emission element.

DON G. BURNSIDE.

